NAME

elf - format of Executable and Linking Format (ELF) files

SYNOPSIS

#include<elf.h>

DESCRIPTION

The header file <elf.h> defines the format of ELF executable binary
files. Amongst these files are normal executable files, relocatable
object files, core files and shared libraries.
An executable file using the ELF file format consists of an ELF header,
followed by a program header table or a section header table, or both.
The ELF header is always at offset zero of the file. The program
header table and the section header table's offset in the file are
defined in the ELF header. The two tables describe the rest of the
particularities of the file.
This header file describes the above mentioned headers as C structures
and also includes structures for dynamic sections, relocation sections
and symbol tables.
The following types are used for N-bit architectures (N=32,64, ElfN
stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t):
ElfN_Addr Unsigned program address, uintN_t
ElfN_Off Unsigned file offset, uintN_t
ElfN_Section Unsigned section index, uint16_t
ElfN_Versym Unsigned version symbol information, uint16_t
Elf_Byte unsigned char
ElfN_Half uint16_t
ElfN_Sword int32_t
ElfN_Word uint32_t
ElfN_Sxword int64_t
ElfN_Xword uint64_t
(Note: The *BSD terminology is a bit different. There Elf64_Half is
twice as large as Elf32_Half, and Elf64Quarter is used for uint16_t.
In order to avoid confusion these types are replaced by explicit ones
in the below.)
All data structures that the file format defines follow the "natural"
size and alignment guidelines for the relevant class. If necessary,
data structures contain explicit padding to ensure 4-byte alignment for
4-byte objects, to force structure sizes to a multiple of 4, etc.
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
#define EI_NIDENT 16
typedef struct {
unsigned char e_ident[EI_NIDENT];
uint16_t e_type;
uint16_t e_machine;
uint32_t e_version;
ElfN_Addr e_entry;
ElfN_Off e_phoff;
ElfN_Off e_shoff;
uint32_t e_flags;
uint16_t e_ehsize;
uint16_t e_phentsize;
uint16_t e_phnum;
uint16_t e_shentsize;
uint16_t e_shnum;
uint16_t e_shstrndx;
} ElfN_Ehdr;
The fields have the following meanings:
e_ident This array of bytes specifies to interpret the file,
independent of the processor or the file's remaining
contents. Within this array everything is named by macros,
which start with the prefix EI_ and may contain values
which start with the prefix ELF. The following macros are
defined:
EI_MAG0 The first byte of the magic number. It must be
filled with ELFMAG0. (0: 0x7f)
EI_MAG1 The second byte of the magic number. It must
be filled with ELFMAG1. (1: 'E')
EI_MAG2 The third byte of the magic number. It must be
filled with ELFMAG2. (2: 'L')
EI_MAG3 The fourth byte of the magic number. It must
be filled with ELFMAG3. (3: 'F')
EI_CLASS The fifth byte identifies the architecture for
this binary:
ELFCLASSNONE This class is invalid.
ELFCLASS32 This defines the 32-bit
architecture. It supports
machines with files and virtual
address spaces up to 4 Gigabytes.
ELFCLASS64 This defines the 64-bit
architecture.
EI_DATA The sixth byte specifies the data encoding of
the processor-specific data in the file.
Currently these encodings are supported:
ELFDATANONE Unknown data format.
ELFDATA2LSB Two's complement, little-endian.
ELFDATA2MSB Two's complement, big-endian.
EI_VERSION The version number of the ELF specification:
EV_NONE Invalid version.
EV_CURRENT Current version.
EI_OSABI This byte identifies the operating system and
ABI to which the object is targeted. Some
fields in other ELF structures have flags and
values that have platform-specific meanings;
the interpretation of those fields is
determined by the value of this byte. E.g.:
ELFOSABI_NONE Same as ELFOSABI_SYSV
ELFOSABI_SYSV UNIX System V ABI.
ELFOSABI_HPUX HP-UX ABI.
ELFOSABI_NETBSD NetBSD ABI.
ELFOSABI_LINUX Linux ABI.
ELFOSABI_SOLARIS Solaris ABI.
ELFOSABI_IRIX IRIX ABI.
ELFOSABI_FREEBSD FreeBSD ABI.
ELFOSABI_TRU64 TRU64 UNIX ABI.
ELFOSABI_ARM ARM architecture ABI.
ELFOSABI_STANDALONE Stand-alone (embedded) ABI.
EI_ABIVERSION
This byte identifies the version of the ABI to
which the object is targeted. This field is
used to distinguish among incompatible versions
of an ABI. The interpretation of this version
number is dependent on the ABI identified by
the EI_OSABI field. Applications conforming to
this specification use the value 0.
EI_PAD Start of padding. These bytes are reserved and
set to zero. Programs which read them should
ignore them. The value for EI_PAD will change
in the future if currently unused bytes are
given meanings.
EI_NIDENT The size of the e_ident array.
e_type This member of the structure identifies the object file
type:
ET_NONE An unknown type.
ET_REL A relocatable file.
ET_EXEC An executable file.
ET_DYN A shared object.
ET_CORE A core file.
e_machine This member specifies the required architecture for an
individual file. E.g.:
EM_NONE An unknown machine.
EM_M32 AT&T WE 32100.
EM_SPARC Sun Microsystems SPARC.
EM_386 Intel 80386.
EM_68K Motorola 68000.
EM_88K Motorola 88000.
EM_860 Intel 80860.
EM_MIPS MIPS RS3000 (big-endian only).
EM_PARISC HP/PA.
EM_SPARC32PLUS
SPARC with enhanced instruction set.
EM_PPC PowerPC.
EM_PPC64 PowerPC 64-bit.
EM_S390 IBM S/390
EM_ARM Advanced RISC Machines
EM_SH Renesas SuperH
EM_SPARCV9 SPARC v9 64-bit.
EM_IA_64 Intel Itanium
EM_X86_64 AMD x86-64
EM_VAX DEC Vax.
e_version This member identifies the file version:
EV_NONE Invalid version.
EV_CURRENT Current version.
e_entry This member gives the virtual address to which the system
first transfers control, thus starting the process. If the
file has no associated entry point, this member holds zero.
e_phoff This member holds the program header table's file offset in
bytes. If the file has no program header table, this
member holds zero.
e_shoff This member holds the section header table's file offset in
bytes. If the file has no section header table this member
holds zero.
e_flags This member holds processor-specific flags associated with
the file. Flag names take the form EF_`machine_flag'.
Currently no flags have been defined.
e_ehsize This member holds the ELF header's size in bytes.
e_phentsize This member holds the size in bytes of one entry in the
file's program header table; all entries are the same size.
e_phnum This member holds the number of entries in the program
header table. Thus the product of e_phentsize and e_phnum
gives the table's size in bytes. If a file has no program
header, e_phnum holds the value zero.
If the number of entries in the program header table is
larger than or equal to PN_XNUM (0xffff), this member holds
PN_XNUM (0xffff) and the real number of entries in the
program header table is held in the sh_info member of the
initial entry in section header table. Otherwise, the
sh_info member of the initial entry contains the value
zero.
PN_XNUM This is defined as 0xffff, the largest number
e_phnum can have, specifying where the actual
number of program headers is assigned.
e_shentsize This member holds a sections header's size in bytes. A
section header is one entry in the section header table;
all entries are the same size.
e_shnum This member holds the number of entries in the section
header table. Thus the product of e_shentsize and e_shnum
gives the section header table's size in bytes. If a file
has no section header table, e_shnum holds the value of
zero.
If the number of entries in the section header table is
larger than or equal to SHN_LORESERVE (0xff00), e_shnum
holds the value zero and the real number of entries in the
section header table is held in the sh_size member of the
initial entry in section header table. Otherwise, the
sh_size member of the initial entry in the section header
table holds the value zero.
e_shstrndx This member holds the section header table index of the
entry associated with the section name string table. If
the file has no section name string table, this member
holds the value SHN_UNDEF.
If the index of section name string table section is larger
than or equal to SHN_LORESERVE (0xff00), this member holds
SHN_XINDEX (0xffff) and the real index of the section name
string table section is held in the sh_link member of the
initial entry in section header table. Otherwise, the
sh_link member of the initial entry in section header table
contains the value zero.
SHN_UNDEF This value marks an undefined, missing,
irrelevant, or otherwise meaningless section
reference. For example, a symbol "defined"
relative to section number SHN_UNDEF is an
undefined symbol.
SHN_LORESERVE This value specifies the lower bound of the
range of reserved indices.
SHN_LOPROC Values greater than or equal to SHN_HIPROC
are reserved for processor-specific
semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC are
reserved for processor-specific semantics.
SHN_ABS This value specifies absolute values for the
corresponding reference. For example,
symbols defined relative to section number
SHN_ABS have absolute values and are not
affected by relocation.
SHN_COMMON Symbols defined relative to this section are
common symbols, such as Fortran COMMON or
unallocated C external variables.
SHN_HIRESERVE This value specifies the upper bound of the
range of reserved indices between
SHN_LORESERVE and SHN_HIRESERVE, inclusive;
the values do not reference the section
header table. That is, the section header
table does not contain entries for the
reserved indices.
An executable or shared object file's program header table is an array
of structures, each describing a segment or other information the
system needs to prepare the program for execution. An object file
segment contains one or more sections. Program headers are meaningful
only for executable and shared object files. A file specifies its own
program header size with the ELF header's e_phentsize and e_phnum
members. The ELF program header is described by the type Elf32_Phdr or
Elf64_Phdr depending on the architecture:
typedef struct {
uint32_t p_type;
Elf32_Off p_offset;
Elf32_Addr p_vaddr;
Elf32_Addr p_paddr;
uint32_t p_filesz;
uint32_t p_memsz;
uint32_t p_flags;
uint32_t p_align;
} Elf32_Phdr;
typedef struct {
uint32_t p_type;
uint32_t p_flags;
Elf64_Off p_offset;
Elf64_Addr p_vaddr;
Elf64_Addr p_paddr;
uint64_t p_filesz;
uint64_t p_memsz;
uint64_t p_align;
} Elf64_Phdr;
The main difference between the 32-bit and the 64-bit program header
lies in the location of the p_flags member in the total struct.
p_type This member of the Phdr struct tells what kind of segment
this array element describes or how to interpret the array
element's information.
PT_NULL The array element is unused and the other
members' values are undefined. This lets the
program header have ignored entries.
PT_LOAD The array element specifies a loadable segment,
described by p_filesz and p_memsz. The bytes
from the file are mapped to the beginning of
the memory segment. If the segment's memory
size p_memsz is larger than the file size
p_filesz, the "extra" bytes are defined to hold
the value 0 and to follow the segment's
initialized area. The file size may not be
larger than the memory size. Loadable segment
entries in the program header table appear in
ascending order, sorted on the p_vaddr member.
PT_DYNAMIC The array element specifies dynamic linking
information.
PT_INTERP The array element specifies the location and
size of a null-terminated pathname to invoke as
an interpreter. This segment type is
meaningful only for executable files (though it
may occur for shared objects). However it may
not occur more than once in a file. If it is
present, it must precede any loadable segment
entry.
PT_NOTE The array element specifies the location and
size for auxiliary information.
PT_SHLIB This segment type is reserved but has
unspecified semantics. Programs that contain
an array element of this type do not conform to
the ABI.
PT_PHDR The array element, if present, specifies the
location and size of the program header table
itself, both in the file and in the memory
image of the program. This segment type may
not occur more than once in a file. Moreover,
it may only occur if the program header table
is part of the memory image of the program. If
it is present, it must precede any loadable
segment entry.
PT_LOPROC Values greater than or equal to PT_HIPROC are
reserved for processor-specific semantics.
PT_HIPROC Values less than or equal to PT_LOPROC are
reserved for processor-specific semantics.
PT_GNU_STACK GNU extension which is used by the
Linux kernel to control the state of the stack
via the flags set in the p_flags member.
p_offset This member holds the offset from the beginning of the file
at which the first byte of the segment resides.
p_vaddr This member holds the virtual address at which the first
byte of the segment resides in memory.
p_paddr On systems for which physical addressing is relevant, this
member is reserved for the segment's physical address.
Under BSD this member is not used and must be zero.
p_filesz This member holds the number of bytes in the file image of
the segment. It may be zero.
p_memsz This member holds the number of bytes in the memory image
of the segment. It may be zero.
p_flags This member holds a bitmask of flags relevant to the
segment:
PF_X An executable segment.
PF_W A writable segment.
PF_R A readable segment.
A text segment commonly has the flags PF_X and PF_R. A
data segment commonly has PF_X, PF_W and PF_R.
p_align This member holds the value to which the segments are
aligned in memory and in the file. Loadable process
segments must have congruent values for p_vaddr and
p_offset, modulo the page size. Values of zero and one
mean no alignment is required. Otherwise, p_align should
be a positive, integral power of two, and p_vaddr should
equal p_offset, modulo p_align.
A file's section header table lets one locate all the file's sections.
The section header table is an array of Elf32_Shdr or Elf64_Shdr
structures. The ELF header's e_shoff member gives the byte offset from
the beginning of the file to the section header table. e_shnum holds
the number of entries the section header table contains. e_shentsize
holds the size in bytes of each entry.
A section header table index is a subscript into this array. Some
section header table indices are reserved: the initial entry and the
indices between SHN_LORESERVE and SHN_HIRESERVE. The initial entry is
used in ELF extensions for e_phnum, e_shnum and e_strndx; in other
cases, each field in the initial entry is set to zero. An object file
does not have sections for these special indices:
SHN_UNDEF This value marks an undefined, missing, irrelevant
or otherwise meaningless section reference.
SHN_LORESERVE This value specifies the lower bound of the range
of reserved indices.
SHN_LOPROC Values greater than or equal to SHN_HIPROC are
reserved for processor-specific semantics.
SHN_HIPROC Values less than or equal to SHN_LOPROC are
reserved for processor-specific semantics.
SHN_ABS This value specifies the absolute value for the
corresponding reference. For example, a symbol
defined relative to section number SHN_ABS has an
absolute value and is not affected by relocation.
SHN_COMMON Symbols defined relative to this section are
common symbols, such as FORTRAN COMMON or
unallocated C external variables.
SHN_HIRESERVE This value specifies the upper bound of the range
of reserved indices. The system reserves indices
between SHN_LORESERVE and SHN_HIRESERVE,
inclusive. The section header table does not
contain entries for the reserved indices.
The section header has the following structure:
typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint32_t sh_flags;
Elf32_Addr sh_addr;
Elf32_Off sh_offset;
uint32_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint32_t sh_addralign;
uint32_t sh_entsize;
} Elf32_Shdr;
typedef struct {
uint32_t sh_name;
uint32_t sh_type;
uint64_t sh_flags;
Elf64_Addr sh_addr;
Elf64_Off sh_offset;
uint64_t sh_size;
uint32_t sh_link;
uint32_t sh_info;
uint64_t sh_addralign;
uint64_t sh_entsize;
} Elf64_Shdr;
No real differences exist between the 32-bit and 64-bit section
headers.
sh_name This member specifies the name of the section. Its value is
an index into the section header string table section, giving
the location of a null-terminated string.
sh_type This member categorizes the section's contents and semantics.
SHT_NULL This value marks the section header as
inactive. It does not have an associated
section. Other members of the section header
have undefined values.
SHT_PROGBITS This section holds information defined by the
program, whose format and meaning are
determined solely by the program.
SHT_SYMTAB This section holds a symbol table. Typically,
SHT_SYMTAB provides symbols for link editing,
though it may also be used for dynamic
linking. As a complete symbol table, it may
contain many symbols unnecessary for dynamic
linking. An object file can also contain a
SHT_DYNSYM section.
SHT_STRTAB This section holds a string table. An object
file may have multiple string table sections.
SHT_RELA This section holds relocation entries with
explicit addends, such as type Elf32_Rela for
the 32-bit class of object files. An object
may have multiple relocation sections.
SHT_HASH This section holds a symbol hash table. An
object participating in dynamic linking must
contain a symbol hash table. An object file
may have only one hash table.
SHT_DYNAMIC This section holds information for dynamic
linking. An object file may have only one
dynamic section.
SHT_NOTE This section holds information that marks the
file in some way.
SHT_NOBITS A section of this type occupies no space in
the file but otherwise resembles SHT_PROGBITS.
Although this section contains no bytes, the
sh_offset member contains the conceptual file
offset.
SHT_REL This section holds relocation offsets without
explicit addends, such as type Elf32_Rel for
the 32-bit class of object files. An object
file may have multiple relocation sections.
SHT_SHLIB This section is reserved but has unspecified
semantics.
SHT_DYNSYM This section holds a minimal set of dynamic
linking symbols. An object file can also
contain a SHT_SYMTAB section.
SHT_LOPROC This value up to and including SHT_HIPROC is
reserved for processor-specific semantics.
SHT_HIPROC This value down to and including SHT_LOPROC is
reserved for processor-specific semantics.
SHT_LOUSER This value specifies the lower bound of the
range of indices reserved for application
programs.
SHT_HIUSER This value specifies the upper bound of the
range of indices reserved for application
programs. Section types between SHT_LOUSER
and SHT_HIUSER may be used by the application,
without conflicting with current or future
system-defined section types.
sh_flags Sections support one-bit flags that describe miscellaneous
attributes. If a flag bit is set in sh_flags, the attribute
is "on" for the section. Otherwise, the attribute is "off"
or does not apply. Undefined attributes are set to zero.
SHF_WRITE This section contains data that should be
writable during process execution.
SHF_ALLOC This section occupies memory during process
execution. Some control sections do not
reside in the memory image of an object file.
This attribute is off for those sections.
SHF_EXECINSTR This section contains executable machine
instructions.
SHF_MASKPROC All bits included in this mask are reserved
for processor-specific semantics.
sh_addr If this section appears in the memory image of a process,
this member holds the address at which the section's first
byte should reside. Otherwise, the member contains zero.
sh_offset This member's value holds the byte offset from the beginning
of the file to the first byte in the section. One section
type, SHT_NOBITS, occupies no space in the file, and its
sh_offset member locates the conceptual placement in the
file.
sh_size This member holds the section's size in bytes. Unless the
section type is SHT_NOBITS, the section occupies sh_size
bytes in the file. A section of type SHT_NOBITS may have a
nonzero size, but it occupies no space in the file.
sh_link This member holds a section header table index link, whose
interpretation depends on the section type.
sh_info This member holds extra information, whose interpretation
depends on the section type.
sh_addralign
Some sections have address alignment constraints. If a
section holds a doubleword, the system must ensure doubleword
alignment for the entire section. That is, the value of
sh_addr must be congruent to zero, modulo the value of
sh_addralign. Only zero and positive integral powers of two
are allowed. Values of zero or one mean the section has no
alignment constraints.
sh_entsize
Some sections hold a table of fixed-sized entries, such as a
symbol table. For such a section, this member gives the size
in bytes for each entry. This member contains zero if the
section does not hold a table of fixed-size entries.
Various sections hold program and control information:
.bss This section holds uninitialized data that contributes to the
program's memory image. By definition, the system
initializes the data with zeros when the program begins to
run. This section is of type SHT_NOBITS. The attribute
types are SHF_ALLOC and SHF_WRITE.
.comment This section holds version control information. This section
is of type SHT_PROGBITS. No attribute types are used.
.ctors This section holds initialized pointers to the C++
constructor functions. This section is of type SHT_PROGBITS.
The attribute types are SHF_ALLOC and SHF_WRITE.
.data This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.data1 This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS. The attribute types are SHF_ALLOC and
SHF_WRITE.
.debug This section holds information for symbolic debugging. The
contents are unspecified. This section is of type
SHT_PROGBITS. No attribute types are used.
.dtors This section holds initialized pointers to the C++ destructor
functions. This section is of type SHT_PROGBITS. The
attribute types are SHF_ALLOC and SHF_WRITE.
.dynamic This section holds dynamic linking information. The
section's attributes will include the SHF_ALLOC bit. Whether
the SHF_WRITE bit is set is processor-specific. This section
is of type SHT_DYNAMIC. See the attributes above.
.dynstr This section holds strings needed for dynamic linking, most
commonly the strings that represent the names associated with
symbol table entries. This section is of type SHT_STRTAB.
The attribute type used is SHF_ALLOC.
.dynsym This section holds the dynamic linking symbol table. This
section is of type SHT_DYNSYM. The attribute used is
SHF_ALLOC.
.fini This section holds executable instructions that contribute to
the process termination code. When a program exits normally
the system arranges to execute the code in this section.
This section is of type SHT_PROGBITS. The attributes used
are SHF_ALLOC and SHF_EXECINSTR.
.gnu.version
This section holds the version symbol table, an array of
ElfN_Half elements. This section is of type SHT_GNU_versym.
The attribute type used is SHF_ALLOC.
.gnu.version_d
This section holds the version symbol definitions, a table of
ElfN_Verdef structures. This section is of type
SHT_GNU_verdef. The attribute type used is SHF_ALLOC.
.gnu.version_r
This section holds the version symbol needed elements, a
table of ElfN_Verneed structures. This section is of type
SHT_GNU_versym. The attribute type used is SHF_ALLOC.
.got This section holds the global offset table. This section is
of type SHT_PROGBITS. The attributes are processor specific.
.hash This section holds a symbol hash table. This section is of
type SHT_HASH. The attribute used is SHF_ALLOC.
.init This section holds executable instructions that contribute to
the process initialization code. When a program starts to
run the system arranges to execute the code in this section
before calling the main program entry point. This section is
of type SHT_PROGBITS. The attributes used are SHF_ALLOC and
SHF_EXECINSTR.
.interp This section holds the pathname of a program interpreter. If
the file has a loadable segment that includes the section,
the section's attributes will include the SHF_ALLOC bit.
Otherwise, that bit will be off. This section is of type
SHT_PROGBITS.
.line This section holds line number information for symbolic
debugging, which describes the correspondence between the
program source and the machine code. The contents are
unspecified. This section is of type SHT_PROGBITS. No
attribute types are used.
.note This section holds information in the "Note Section" format.
This section is of type SHT_NOTE. No attribute types are
used. OpenBSD native executables usually contain a
.note.openbsd.ident section to identify themselves, for the
kernel to bypass any compatibility ELF binary emulation tests
when loading the file.
.note.GNU-stack
This section is used in Linux object files for declaring
stack attributes. This section is of type SHT_PROGBITS. The
only attribute used is SHF_EXECINSTR. This indicates to the
GNU linker that the object file requires an executable stack.
.plt This section holds the procedure linkage table. This section
is of type SHT_PROGBITS. The attributes are processor
specific.
.relNAME This section holds relocation information as described below.
If the file has a loadable segment that includes relocation,
the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is
supplied by the section to which the relocations apply. Thus
a relocation section for .text normally would have the name
.rel.text. This section is of type SHT_REL.
.relaNAME This section holds relocation information as described below.
If the file has a loadable segment that includes relocation,
the section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. By convention, "NAME" is
supplied by the section to which the relocations apply. Thus
a relocation section for .text normally would have the name
.rela.text. This section is of type SHT_RELA.
.rodata This section holds read-only data that typically contributes
to a nonwritable segment in the process image. This section
is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.rodata1 This section holds read-only data that typically contributes
to a nonwritable segment in the process image. This section
is of type SHT_PROGBITS. The attribute used is SHF_ALLOC.
.shstrtab This section holds section names. This section is of type
SHT_STRTAB. No attribute types are used.
.strtab This section holds strings, most commonly the strings that
represent the names associated with symbol table entries. If
the file has a loadable segment that includes the symbol
string table, the section's attributes will include the
SHF_ALLOC bit. Otherwise the bit will be off. This section
is of type SHT_STRTAB.
.symtab This section holds a symbol table. If the file has a
loadable segment that includes the symbol table, the
section's attributes will include the SHF_ALLOC bit.
Otherwise the bit will be off. This section is of type
SHT_SYMTAB.
.text This section holds the "text", or executable instructions, of
a program. This section is of type SHT_PROGBITS. The
attributes used are SHF_ALLOC and SHF_EXECINSTR.
String table sections hold null-terminated character sequences,
commonly called strings. The object file uses these strings to
represent symbol and section names. One references a string as an
index into the string table section. The first byte, which is index
zero, is defined to hold a null byte ('\0'). Similarly, a string
table's last byte is defined to hold a null byte, ensuring null
termination for all strings.
An object file's symbol table holds information needed to locate and
relocate a program's symbolic definitions and references. A symbol
table index is a subscript into this array.
typedef struct {
uint32_t st_name;
Elf32_Addr st_value;
uint32_t st_size;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
} Elf32_Sym;
typedef struct {
uint32_t st_name;
unsigned char st_info;
unsigned char st_other;
uint16_t st_shndx;
Elf64_Addr st_value;
uint64_t st_size;
} Elf64_Sym;
The 32-bit and 64-bit versions have the same members, just in a
different order.
st_name This member holds an index into the object file's symbol
string table, which holds character representations of the
symbol names. If the value is nonzero, it represents a
string table index that gives the symbol name. Otherwise,
the symbol table has no name.
st_value This member gives the value of the associated symbol.
st_size Many symbols have associated sizes. This member holds zero
if the symbol has no size or an unknown size.
st_info This member specifies the symbol's type and binding
attributes:
STT_NOTYPE The symbol's type is not defined.
STT_OBJECT The symbol is associated with a data object.
STT_FUNC The symbol is associated with a function or other
executable code.
STT_SECTION The symbol is associated with a section. Symbol
table entries of this type exist primarily for
relocation and normally have STB_LOCAL bindings.
STT_FILE By convention, the symbol's name gives the name
of the source file associated with the object
file. A file symbol has STB_LOCAL bindings, its
section index is SHN_ABS, and it precedes the
other STB_LOCAL symbols of the file, if it is
present.
STT_LOPROC This value up to and including STT_HIPROC is
reserved for processor-specific semantics.
STT_HIPROC This value down to and including STT_LOPROC is
reserved for processor-specific semantics.
STB_LOCAL Local symbols are not visible outside the object
file containing their definition. Local symbols
of the same name may exist in multiple files
without interfering with each other.
STB_GLOBAL Global symbols are visible to all object files
being combined. One file's definition of a
global symbol will satisfy another file's
undefined reference to the same symbol.
STB_WEAK Weak symbols resemble global symbols, but their
definitions have lower precedence.
STB_LOPROC This value up to and including STB_HIPROC is
reserved for processor-specific semantics.
STB_HIPROC This value down to and including STB_LOPROC is
reserved for processor-specific semantics.
There are macros for packing and unpacking the
binding and type fields:
ELF32_ST_BIND(info) or ELF64_ST_BIND(info)
extract a binding from an st_info value.
ELF32_ST_TYPE(info) or ELF64_ST_TYPE(info)
extract a type from an st_info value.
ELF32_ST_INFO(bind, type) or ELF64_ST_INFO(bind,
type)
convert a binding and a type into an st_info
value.
st_other This member defines the symbol visibility.
STV_DEFAULT Default symbol visibility rules.
STV_INTERNAL Processor-specific hidden class.
STV_HIDDEN Symbol is unavailable in other modules.
STV_PROTECTED Not preemptible, not exported.
There are macros for extracting the visibility type:
ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)
st_shndx Every symbol table entry is "defined" in relation to some
section. This member holds the relevant section header table
index.
Relocation is the process of connecting symbolic references with
symbolic definitions. Relocatable files must have information that
describes how to modify their section contents, thus allowing
executable and shared object files to hold the right information for a
process's program image. Relocation entries are these data.
Relocation structures that do not need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
} Elf32_Rel;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
} Elf64_Rel;
Relocation structures that need an addend:
typedef struct {
Elf32_Addr r_offset;
uint32_t r_info;
int32_t r_addend;
} Elf32_Rela;
typedef struct {
Elf64_Addr r_offset;
uint64_t r_info;
int64_t r_addend;
} Elf64_Rela;
r_offset This member gives the location at which to apply the
relocation action. For a relocatable file, the value is
the byte offset from the beginning of the section to the
storage unit affected by the relocation. For an executable
file or shared object, the value is the virtual address of
the storage unit affected by the relocation.
r_info This member gives both the symbol table index with respect
to which the relocation must be made and the type of
relocation to apply. Relocation types are processor
specific. When the text refers to a relocation entry's
relocation type or symbol table index, it means the result
of applying ELF_[32|64]_R_TYPE or ELF[32|64]_R_SYM,
respectively, to the entry's r_info member.
r_addend This member specifies a constant addend used to compute the
value to be stored into the relocatable field.
The .dynamic section contains a series of structures that hold relevant
dynamic linking information. The d_tag member controls the
interpretation of d_un.
typedef struct {
Elf32_Sword d_tag;
union {
Elf32_Word d_val;
Elf32_Addr d_ptr;
} d_un;
} Elf32_Dyn;
extern Elf32_Dyn _DYNAMIC[];
typedef struct {
Elf64_Sxword d_tag;
union {
Elf64_Xword d_val;
Elf64_Addr d_ptr;
} d_un;
} Elf64_Dyn;
extern Elf64_Dyn _DYNAMIC[];
d_tag This member may have any of the following values:
DT_NULL Marks end of dynamic section
DT_NEEDED String table offset to name of a needed library
DT_PLTRELSZ Size in bytes of PLT relocs
DT_PLTGOT Address of PLT and/or GOT
DT_HASH Address of symbol hash table
DT_STRTAB Address of string table
DT_SYMTAB Address of symbol table
DT_RELA Address of Rela relocs table
DT_RELASZ Size in bytes of Rela table
DT_RELAENT Size in bytes of a Rela table entry
DT_STRSZ Size in bytes of string table
DT_SYMENT Size in bytes of a symbol table entry
DT_INIT Address of the initialization function
DT_FINI Address of the termination function
DT_SONAME String table offset to name of shared object
DT_RPATH String table offset to library search path
(deprecated)
DT_SYMBOLIC Alert linker to search this shared object before
the executable for symbols
DT_REL Address of Rel relocs table
DT_RELSZ Size in bytes of Rel table
DT_RELENT Size in bytes of a Rel table entry
DT_PLTREL Type of reloc the PLT refers (Rela or Rel)
DT_DEBUG Undefined use for debugging
DT_TEXTREL Absence of this indicates no relocs should apply
to a nonwritable segment
DT_JMPREL Address of reloc entries solely for the PLT
DT_BIND_NOW Instruct dynamic linker to process all relocs
before transferring control to the executable
DT_RUNPATH String table offset to library search path
DT_LOPROC Start of processor-specific semantics
DT_HIPROC End of processor-specific semantics
d_val This member represents integer values with various
interpretations.
d_ptr This member represents program virtual addresses. When
interpreting these addresses, the actual address should be
computed based on the original file value and memory base
address. Files do not contain relocation entries to fixup
these addresses.
_DYNAMIC Array containing all the dynamic structures in the .dynamic
section. This is automatically populated by the linker.

NOTES

ELF first appeared in System V. The ELF format is an adopted standard.
The extensions for e_phnum, e_shnum and e_strndx respectively are Linux
extensions. Sun, BSD and AMD64 also support them; for further
information, look under SEE ALSO.